In plain words
The fruit of good action is said to be pure and sāttvika; the fruit of rajas is suffering, and the fruit of tamas is ajñāna.
Word for word
Of action well done they declare the fruit sāttvika, stainless; but of rajas the fruit [is] suffering; ajñāna [is] the fruit of tamas.
कर्मणः सुकृतस्याऽऽहुः सात्त्विकं निर्मलं फलम्रजसस्तु फलं दुःखमज्ञानं तमसः फलम्
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam
अतीतश्लोकार्थस्यैव सङ्क्षेपः उच्यते —
कर्मणः सुकृतस्य सात्त्विकस्य इत्यर्थः, आहुः शिष्टाः सात्त्विकम् एव निर्मलं फलम् इति । रजसस्तु फलं दुःखं राजसस्य कर्मणः इत्यर्थः, कर्माधिकारात् फलम् अपि दुःखम् एव, कारणानुरूप्यात् राजसमेव । तथा अज्ञानं तमसः तामसस्य कर्मणः अधर्मस्य पूर्ववत् ॥ १६ ॥
In plain words
What comes next is simply a summary of what was just said —
A good action, one born of sattva, bears a fruit the wise call pure and clean, sāttvika through and through. But the fruit of rajas is sorrow, the fruit of an action driven by passion — because whatever an action is, its fruit falls squarely under its rule, and that fruit is bound to be sorrowful, matching its cause exactly: it is rajas, plain and simple. And tamas works the same way: its fruit is ignorance, the fruit of a tāmasa action, an unrighteous one, following the same reasoning as before.
Close to the Sanskrit
A summary of the meaning of the preceding verse alone is [now] stated —
Of an action well done, [i.e.] of a sāttvika one — that is the meaning — the learned say the fruit is sāttvika alone, stainless. But of rajas, the fruit is sorrow, [i.e.] of a rājasa action — that is the meaning; because of the very jurisdiction of action, the fruit too is sorrow indeed, being in conformity with its cause, [it is] rājasa indeed. Similarly, ignorance is the fruit of tamas, [i.e.] of a tāmasa action, of unrighteousness — as before.